1. Field of the Invention
This invention relates to a visual axis detection apparatus, and particularly to a visual axis detection apparatus in an optical apparatus such as a camera designated to detect the axis in the direction of a viewing point being observed by an observer (photographer) on an observation surface (a focusing screen) on which is formed an object image by a photo-taking system, i.e., the so-called visual axis, by the utilization of a corneal reflection image obtained when the surface of an eyeball of the observer is illuminated.
2 Related Background Art
There have heretofore been proposed various apparatuses for detecting which position on an observation surface the observer is observing, i.e., detecting the so-called visual axis.
For example, in Japanese Laid-Open Patent Application No. 61-172552, a parallel light beam from a light source is projected onto the front eye part of an eyeball of the observer and the visual axis is formed by the utilization of the image positions of the corneal reflection image by the reflected light from the cornea and a pupil. FIGS. 10A and 10B of the accompanying drawings illustrate the principle of a visual axis detecting method, FIG. 10A being a schematic view of a visual axis detecting optical system, and FIG. 10B showing the intensity of an output signal from a photoelectric element array 6.
In FIG. 10A, the reference numeral 5 designates a light source such as a light emitting diode which emits infrared light not sensed by the observer. The light source 5 is disposed on the focal plane of a light projecting lens 3.
The infrared light emitted from the light source 5 is collimated by the light projecting lens 3, is reflected by a half mirror 2 and illuminates the cornea 21 of an eyeball 201. At this time, the corneal reflection image (Purkinje image) d by part of the infrared light reflected by the surface of the cornea is transmitted through the half mirror 2, is condensed by a light receiving lens 4 and is re-imaged at a position Zd' on the photoelectric element array 6.
Also, a light beam from the end portions a and b of an iris 23 form the images of these end portions a and b at positions Za' and Zb' on the photoelectric element array 6 through the half mirror 2 and the light receiving lens 4. Where the rotation angle .theta. of the optical axis OA2 of the eyeball with respect to the optical axis of the light receiving lens 4 (the optical axis OA1) is small, when the Z coordinates of the end portions a and b of the iris 23 are Za, Zb, the coordinates Zc of the center position c of the iris 23 are expressed as EQU Zc.perspectiveto.(Za+Zb)/2.
Also, when the Z coordinates of the position d at which the corneal reflection image is created and the distance between the center of curvature 0 of the cornea 21 and the center C of the pupil 24 is L.sub.OC, the rotation angle .theta. of the optical axis OA2 of the eyeball substantially satisfies the following relational expression: EQU L.sub.OC * SIN.theta..perspectiveto.Zc-Zd. (1)
Here, the Z coordinates Zd of the position d of the corneal reflection image and the Z coordinates Zo of the center of curvature 0 of the cornea 21 are coincident with each other. Therefore, in calculation means 9, the positions of particular points (the reflected image d of the cornea and the end portions a and b of the iris) projected onto the surface of the photoelectric element array 6 as shown in FIG. 10B are detected, whereby the angle of rotation .theta. of the optical axis OA2 of the eyeball can be found. At this time, expression (1) is rewritten into as ##EQU1## where .beta. is a magnification determined by the distance L1 between the position d at which the corneal reflection image is created and the light receiving lens 4 and the distance L0 between the light receiving lens 4 and the photoelectric element array 6, and usually is a substantially constant value.
This is effective, for example, for saving the trouble of selecting sand inputting one of distance measuring points provided not only at the center of the image field but also at a plurality of locations in the image field in the automatic focus detecting apparatus of a camera when the observer attempts to select one of those distance measuring points and perform automatic focus detection, and regarding that point being observed by the observer as a distance measuring point, and automatically selecting the distance measuring point to thereby accomplish automatic focus detection.
In a visual axis detection apparatus according to the prior art, an image sensor 7 (image pickup means), comprising a plurality of photoelectric element arrays 6, such as CCDs, is used as light receiving means.
FIG. 11A of the accompanying drawings is an illustration showing the corneal reflection image of an eyeball and the reflected image of the iris formed on the surface of such an image sensor 7, and FIG. 11B of the accompanying drawings is an illustration of an output signal from one of the photoelectric element arrays 6 of FIG. 11A.
In these figures, the positions of the corneal reflection image Zd' and the marginal points Za' and Zb' of the iris 23 and the pupil 24 are detected by a photoelectric element array Y1.
Generally, a method whereby an output signal from a photoelectric element array is A/D-converted, whereafter it is subjected to a differential process is known as a positive detecting method using an image sensor.
As shown in FIG. 11B, the output signal from the photoelectric element array for the corneal reflection image Zd' is very great and can be detected relatively easily. On the other hand, the output signals from the photoelectric element arrays for the marginal points Za' and Zb' of the iris 23 and the pupil 24 are weak, and the reflectivity of the iris 23 differs from person to person. Therefore, a ghost or a flare can enter a detecting optical system, and in the case of a person wearing glasses, reflected images from the glasses provide a noise component, and this reduces the detection accuracy of the marginal points Za' and Zb'.
For example, when an attempt is made to take the differential value of the output signals of FIG. 11B and detect it by a predetermined threshold value, a plurality of points which can be regarded as the marginal point P1 of the iris 23 and the pupil 24 will come into existence.
This has led to a case where the reliability of the detection position becomes low and the visual. axis is detected erroneously. To avoid this, it is necessary to use new software, and this has resulted in the problem that the calculation process becomes complex and further, the calculation speed becomes low and it becomes difficult to effect visual axis detection in real time.